As is well known in the art, the successful implementation of intelligent (self-adjustable) optical imaging systems requires devices capable of controllably changing their optical properties. One of the most important optical functions required to be adjustable is the focusing capacity and the focusing distance. Those properties are essential, for example, for the manufacturing of high quality cell phone cameras, storage/reading systems, adjustable glass of contact and other vision systems.
In modern high performance optical imaging systems the optical zoom is obtained by the use of a mechanical movement. This implies that those imaging systems are relatively big (to accommodate, e.g., a step motor), heavy and generally have a slow zoom (at the order of second).
Several approaches have been explored to replace the electro mechanical zoom. A variable-focus liquid lens has been demonstrated using changing aperture [H. Ren, S-T Wu, Variable-focus liquid lens by changing aperture, Appl. Phys. Lett., v. 86, 211107, 2005]. Electro wetting effect in conductive immiscible liquids has been also used to obtain focus tuning [S. Kuiper, B. H. W. Hendriks, Variable-focus liquid lens for miniature cameras, Appl. Phys. Letters, V. 85, No. 7, 1128, 2004]. However in both approaches the variable voltage, delivered to the cell, causes mechanical changes of the shape. So even, if there are no free-space distinct moving components, the focus variation is still based on the mechanical movement. This is highly undesirable since the operation of such systems is vulnerable to environmental vibrations and temperature changes.
It is well known that LCs may provide huge electrically controlled refractive index changes [L. M. Blinov, V. G. Chigrinov, Electrooptic effects in Liquid Crystal Materials, Springer-Verlag, N.Y. 459 pp, 1994.]. However, the focus tuning (which is required for optical zoom) requires the generation of spatially varying refractive index changes in LCs. This, in turn, usually requires either a spatially non uniform LC layer (for example, a lens that is submerged in the LC cell or a gradient polymer network stabilized LC [V. V. Presnyakov, K. E. Asatryan, and T. Galstian, A. Tork, Tunable polymer-stabilized liquid crystal microlens, Optics Express, Vol. 10, No. 17, August 26, pp. 865-870, 2002]) or a spatially varying electric field.
A schematic representation of a spatially non-uniform LC layer is shown in the FIG. 1a (PRIOR ART), [L. G. Commander, S. E. Day, D. R. Selviah, Variable focal length microlenses, Optics Communications, V. 177, pp. 157-170, 2000] where the LC (1) is sandwiched between two transparent glass substrates (2) covered by uniform transparent electrodes (4). The substrates (2) are separated by a gap of a thickness d filled with LC. Inside the LC cell there is a transparent optical material (3) of hemi-spherical profile with refractive index nm. The effective index of refraction neff of the LC (defined by the orientation of its director n that is the average direction of long molecular axes) may be changed with respect to nm by applying an electric voltage V across the LC layer. The relative contrast of refractive index neff(V)-nm may thus be changed resulting in a change of focal distance F(V) of the whole system. However, the LC orientation being typically obtained by mechanical rubbing, this method is very difficult to apply in industrial manufacturing. Also disclination lines are easily appearing upon the application of electric voltage. These are the reasons why we shall not analyze further such solutions.
Another method of obtaining non-uniform LC orientation is the introduction of non uniform polymer network that is stabilizing the LC matrix [T. Galstian, V. Presniakov, A. Tork, K. Asatryan, Electrically variable focus polymer-stabilized liquid crystal lens, US patent application publication No. 20050018127-A1]. However, relatively high light scattering (due to small-size non uniform LC reorientation) makes this method less interesting for practical applications.
The simplest (not from the manufacturing point of view)method of obtaining a spatially varying electric field is the use of multiple (more than 2) transparent electrodes (such as Indium Tin Oxide/ITO) distributed on the LC cell substrates. [S. T. Kowel, P. G. Kornreich, D. S. Cleverly, Adaptive liquid crystal lens, U.S. Pat. No. 4,572,616, 1986 (filed August 1982)] and [N. A. Riza, M. C. DeJule, Three-terminal adaptive nematic liquid-crystal lens device, Opt. Lett. 19, pp. 1013-1015, 1994.] However, the fabrication of such structures requires sub-micrometer precision, their electrical driving requires rather complex electronic micro processing and their operation is degraded by light diffraction and scattering.
Combination of planar and curved electrodes has been described in Ref. [Liquid Crystal Lens with Spherical Electrode, B. Wang, M. Ye, M. Honma, T. Nose, S. Sato, Jpn. J. Appl. Phys. Vol. 41(2002), pp. L1232-L1233, Part 2, No. 11A, 1 November.], which allows the use of standard (transparent) electrodes and LC cells having two planar internal surfaces (FIG. 1b, PRIOR ART). The non uniform (centrally symmetric) electric field is obtained thanks to the geometrical lens-like form (31) of the “external” curved surface which is coated by the upper electrode (4). In fact, the planar LC (1) layer is sandwiched between two glass substrates (2). The planar ITO electrode is coated on the bottom (plane) surface of one substrate, while the second electrode is fabricated on the top of the curved zone (31). Such structure is difficult to fabricate and has a 0-voltage lensing property (what we call “action-at-0-voltage”), which may cause problems if an unexpected voltage failure happens.
This 0-voltage lensing may be eliminated by using an additional polymer layer that is placed over the curved and ITO-coated surface and which has flat upper surface [H. Ren, Y. H. Fan, S. Gauza, S. T. Wu, Tunable-focus flat liquid crystal spherical lens, Applied Phys. Lett., V. 84, No. 23, pp. 4789-4791, (2004).]. This approach, in fact, allows to permanently “hidden” the 0-voltage lensing effect (providing “no-action-at-0-voltage”) while its fabrication remains complicated and costly.
Similar solution has been described in Ref. [U.S. Pat. No. 6,859,333: H. Ren, Y.-H Fan, S.-T. Wu “Adaptive liquid crystal lenses”, February 2005, filled January 2004)] for the fabrication of diffractive tunable lenses. This is an adaptive optical lens device composed of at least two planar substrates and at least one homogeneous nematic liquid crystal (NLC) layer. One planar substrate has a spherical or annular ring-shaped Fresnel grooved transparent electrode within it, the other has a transparent electrode coated on its inner surface. The thickness of the NLC layer is uniform. When a voltage is applied across the LC layer, a centro-symmetrical gradient distribution of refractive index within LC layer occurs. Therefore, the LC layer causes light to focus. By controlling the applied voltage, the focal length of the lens is continuously tunable.
While the flat internal surfaces of the LC cell are easier to fabricate, the complex-curved geometry of “external” surfaces and the electrode deposition on those surfaces make difficult the fabrication of such lenses.
Various geometrical solutions have been proposed to avoid the use of multiple and complex electrodes. One of them is based on the use of a two-dimensional geometrical form of electrodes. For example, hole patterned electrode has been used in Refs. [M. Ye, S. Sato, Jpn. J. Appl. Phys., V. 41, (2002), L571; U.S. Pat. No. 6,768,536: D. Okuwaki, S. Sato “Liquid crystal microlens” July 2004, filled November 2002);] and in Ref. [B. Wang, M. Ye, S. Sato, Liquid-crystal lens with stacked structure of liquid-crystal layers, Optics Communications, 250 (2005), pp. 266-273]. The basic idea of this approach is described in the FIG. 2a (PRIOR ART). This is a rather standard cell with LC (1) sandwiched between two substrates (2) and one of substrates (the bottom one) is coated by an ITO (4). However, there is a hole (5) in the upper electrode (41). The application of the voltage between (4) and (41) generates a centrally symmetric electric field (42), which reorients the LC director n in a spatially nonuniform (centrally symmetric) way. This, in turn, generates neff(V,x) that has a corresponding form in the space x. The main drawback of this structure is the necessity to use very thick LC layers (large d) to be able to obtain the desired spatial profile of the electric field in the LC layer and maintain good optical quality of the lens (particularly to avoid optical aberrations).
An improved version of this approach ([B. Wang, M. Ye, S. Sato, Liquid-crystal lens with stacked structure of liquid-crystal layers, Optics Communications, 250 (2005), pp. 266-273], see FIG. 2b, PRIOR ART.) contains multiple transparent substrates (2), a pair of uniform transparent electrodes provided on the bottom side of the upper substrate (40) and on the upper surface of the lower substrate (4). An intermediate electrode (41) with a circular hole (5) is introduced between those electrodes (4). To obtain an acceptable lens-like refractive index gradient (for low aberrations) and disclination-free LC reorientation, the distance between the hole patterned electrode (41) and the uniform electrode 4 must be more than 500-1300 um (for lenses with 4-5 mm of diameter), which leads to the necessity of high voltages. The LC generates a lens-like structure when first high and fixed auxiliary voltage V0 (at the order of 150 V) and then a control voltage Vc (at the order of 175V) are applied simultaneously (between the electrode 4 and electrodes 41 and 40, respectively). Then, after a certain optimal delay T the control voltage Vc is decreased to the desired value). High voltages and complex dynamics required make this method rather difficult to implement in practice. Complex, multiple cell solutions based on the same approach are increasing the manufacturing cost and driving complexity.
So-called “Modal-controlled” liquid crystal lens has been demonstrated in Ref. [A. F. Naumov, M. Yu. Loktev, I. R. Guralnik, G. Vdovin, “Liquid-crystal adaptive lenses with modal control”, Opt. Lett. 23, 992-994, 1998; G. D. Love, A. F. Naumov, “Modal liquid crystal lenses”, Liq. Cryst. Today, 10(1), pp. 1-4, 2000; M. Yu. Loktev, V. N. Belopukhov, F. L. Vladimirov, G. V Vdovin, G. D. Love, A. F. Naumov, Wave front control systems based on modal liquid crystal lenses, Review of Scientific Instruments, V. 71, No. 9, pp. 3290-3297, (2000).]. High resistance annular electrode (40) is used here (FIG. 2c, PRIOR ART) in contrast to the previous hole-patterned electrode. The voltage is applied between the uniform electrode (4) of the bottom substrate and the annular electrode (40) of the upper substrate. Thanks to the complex impendence, formed by the highly resistant electrode (40) and the LC layer (1), the distribution of the rms voltage (and corresponding control field (42) and (43)) across the cell is centrally symmetric but non-uniform (with a center coinciding with the center of the electrode 40). The fields (42) and (43) are described by Bessel functions, and the voltage-optical retardance dependency is approximately an inversion logarithmic function. The problems of this approach are the strong light absorption by the highly resistant electrode (40), the optical aberrations (since, if a voltage of arbitrary magnitude and phase is applied to the cell, then the resulting phase distribution will be far from parabolic) and complexity of the electrical control (voltage and frequency).
In contrast with all previous examples (where the non uniform electric field is obtained by the use of geometrical form of curved or hole patterned electrodes or via the impedance induced gradient), an elegant solution was proposed, which uses the gradient of the dielectric permittivity of materials at low frequency (e.g., 1 kHz) electric field ∈DC (here called “DC” to note the driving electric field). Namely, an intermediate layer (3) is inserted between two control electrodes (4) to generate the desired gradient of the driving electric field [B. Wang, M. Ye, S. Sato, Lens of electrically controllable focal length made by a glass lens and liquid crystal layers, Applied Optics, V. 43, No. 17, pp. 3420-3425, 2004.], FIG. 3 (PRIOR ART). In fact, the intermediate layer (3) is composed of glass (with ∈(g)DC) and has spatially non uniform thickness. The remaining part (7) of the intermediate space is filled by air, with ∈(a)DC≈1. The application of the low frequency electric voltage (through electrodes 4) generates a spatially non uniform electric field inside the LC cell, because of the non-uniformity of the dielectric permittivity of the intermediate media ∈(a)DC>∈(a)DC. The electric field 43 in the central part of the cell will thus be different (weaker) from the electric field 42 near to the border. In the particular case of FIG. 3 (PRIOR ART) two LC cells are used to enhance the effect. The advantage of this approach is that the desired spatial form (gradient) may be obtained by the use of the intermediate material of appropriate form (such as a lens).
However, three major problems remain to be solved in this approach too. One of them is the inherent 0-voltage lensing effect (“action-at-0-voltage”). The second problem is related to the necessity of having multiple antireflection coatings to avoid high optical losses of this geometry due to Fresnel reflections on multiple glass-air surfaces (since at optical frequencies the refractive index is quite different for glass ng≈1.5 and air na≈1). Finally, the achievable contrast of electric field (and thus of the neff(V,x)) is severely limited because of the maximum achievable contrast between ∈(g)DC (which can be varied from 3.8 to 14.5 depending on the type of glass) and ∈(a)DC≈1 (in fact, one could use a high n material lens, but it would generate significant Fresnel losses).
Accordingly, low loss, efficient and electrically tunable focal optical devices remain highly desirable.